RCCI relies on the blending of two fuels, each with different ignition-reactivity characteristics, early in the combustion cycle. The port injection of the gasoline fuel (which has lower reactivity) promotes air/fuel mixing before the higher-reactivity diesel fuel is injected directly into the combustion chamber for autoignition. Natural gas and ethanol are also being investigated as alternatives to gasoline.

“By changing the injection timing and quantities of both fuels, optimal combustion can be achieved at many engine operating conditions,” Reed Hanson, Ph.D, Research Associate at the University of Wisconsin-Madison Engine Research Center (ERC), explained to Automotive Engineering.

Hanson is part of the RCCI development team led by Dr. Rolf Reitz, mechanical engineering professor and director of ERC’s Diesel Engine Consortium. The researchers are essentially developing a recipe for mixing two fuels using key parameters including the engine’s operating speed.

“If you operate at high engine speed, you’ll want the overall mix to be more reactive because there is less time for it to combust,” Dr. Reitz said in a 2011 SAE Momentum interview (http://articles.sae.org/9298).

RCCI as it is being developed by UW-Madison involves three patent and patent-pending technologies relating to in-cylinder blending, stratification and the compression-ignition combustion process. Last January, the project reached a milestone when a crossover utility became the first vehicle fitted with RCCI technology.

Potential for reduced aftertreatment

The demonstrator vehicle is a 2009 Saturn Vue that was reconfigured after its stint as the UW-Madison Hybrid Vehicle Team’s entry for the 2008-2010 EcoCAR competition. For that role, the Vue was converted to an extended-range EV powered by a 60-kW e-motor with a turbocharged Weber 750-cm3 ICE running on E85 to drive the front wheels, and a 55-kW motor powering the rear wheels. A lithium-ion battery pack was used for all-electric driving.

For the RCCI project the Vue was transformed again. A 1.9-L GM diesel modified for RCCI operation replaced the Weber engine, with a 90-kW generator in place of the previous 60-kW unit. A 75-kW traction motor now provides the rear-axle drive, and the battery pack is rated at 300-V.

The dual-fuel strategy required the installation of a second fuel tank, fuel pump, injectors and related components. The intake manifold was modified to enable the gasoline injectors to work with the diesel engine. The retrofit process also included the installation of custom-designed pistons for RCCI operation and a National Instruments Drivven controller for optimal RCCI performance from the gasoline injectors.

Early project research has shown that RCCI technology can effectively increase vehicle fuel efficiency by up to 20% compared to a baseline conventional diesel, as well as the potential for meeting 2016 U.S., EU, and Japanese emission standards.

“RCCI promotes low NOx and soot emissions by operating at low peak combustion temperatures and lean air/fuel ratios. By doing this, the engine is operated in regions where these pollutants are not formed,” said Hanson. “Operating lean and at low combustion temperatures also reduces heat transfer losses, which increases the thermal efficiency,” he added.

Biofuel next steps

In-vehicle evaluations of RCCI technology are providing additional perspective to the lab-based findings. “We are learning how RCCI is able to adapt to real-world boundary conditions, namely changing coolant and intake temperatures. We also are learning how to relate engine dynamometer results with chassis dynamometer tests as the emissions targets are very different from each other,” Hanson noted.

The RCCI research team’s work is aimed at making the technology commercially viable for various applications, such as passenger vehicles, medium- and heavy-duty trucks and buses, construction and other off-road vehicles as well as locomotives, marine vessels, and generator sets.

“Over the last few months, we have operated RCCI under changing speed and load conditions. Showing that RCCI can operate in these transient conditions is a major step toward mass production applications,” explained Hanson.

The next steps for researchers include the testing of various biofuels, in particular E20 and B20. “We also will look at replacing diesel fuel with a gasoline/cetane number improver (an additive) mixture to allow for one main fuel tank instead of two tanks. And we will investigate vehicle operation with more transient operating conditions as a way to further refine the RCCI technology,” he said.